Silver halide photography usually involves the exposure of silver halide with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. Thus, when silver halide is to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element or infrared radiation in an infrared-sensitive element, a spectral sensitizing dye is required. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds) that are adsorbed to the silver halide. They absorb light or radiation of a particular wavelength and transfer the energy to the silver halide to form the latent image, thus effectively rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity. Sensitizing dyes can also be used to augment the sensitivity of silver halide in the blue region of the spectrum.
During processing of color photographic materials, the silver halide is removed from the material. With black and white materials, the silver halide that was not exposed is removed. In either case, it is desirable to remove the sensitizing dye as well. Sensitizing dye that is not removed tends to cause retained dye stain, which adversely affects the image recorded in the photographic material. The problem of retained sensitizing dye stain is further aggravated by the increasing use of tabular grain emulsions and high chloride emulsions. Tabular grain emulsions have a high surface area per mole of silver, which can lead to higher levels of sensitizing dye and thus, higher levels of retained dye stain. High chloride emulsions necessitate the use of sensitizing dyes having enhanced adsorption to silver halide, and are also often subjected to rapid processing, which can aggravate dye stain problems.
Most photographic films needing green sensitization require a high degree of sensitivity at the wavelengths of the mid-green region of the spectrum (540-555 nm, the maximum sensitivity range of the human eye) for adequate speed, color separation, and color reproduction. Also, absorbance of light in the mid-green region is important for radiographic elements employing phosphor screens which emit light in this region. Benzimidazolocarbocyanine, oxacarbocyanine, and benzimidazolooxacarbocyanine dyes are all well known classes of spectral sensitizing dyes which absorb light in the green region of the spectrum. Species of these classes of dyes are disclosed in, for example, U.S. Pat. Nos. 4,425,425 and 4,425,426 (Reexamination Certificate 907) of Abott et al., U.S. Pat. No. 4,510,235 of Ukai et al., U.S. Pat. No. 4,801,526 of Yoshida et al., and U.S. Pat. No. 4,837,140 of Ikeda et al.
Benzimidazolocarbocyanine dyes are very efficient at utilizing light energy and their high basicity allows them to be protonated and removed in processes which use acidic solutions, leaving low residual stain. These dyes function best as J-aggregates on the silver halide grain surface. Such benzimidazolocarbocyanine aggregates, however, generally absorb light at 560 to 590 nm, the long green region of the spectrum. As such, it has been heretofore necessary to use a different class of dyes, e.g. the oxacarbocyanines or benzimidazolo-oxacarbocyanines, for sensitization in the mid-green region. These dyes, however, being less basic tend to leave unacceptably high levels of retained dye after processing.
Another feature of many benzimidazolo-carbocyanines is their relatively low oxidation potential which may lead to poor storage stability of the photographic film or paper in which they have been incorporated caused by the oxidative instability of the sensitizing dye. This poor keeping may be manifested as an increase in fog and/or a loss of photographic speed with storage or incubation of the photographic material.
It is thus an object of the invention to provide sensitizing dyes that aggregate and sensitize efficiently in the 540 to 555 nm region of the spectrum and that leave very low levels of residual dye stain in photographic elements after processing. A further object of the invention is to provide such dyes which are also very stable upon storage.